Last time I checked, a .91 4-stroke weighs less than a .92 2-stroke. I have a couple of examples of each. Here are the weights, with muffler:
Magnum .91FS: 22.5 oz
Magnum .70FS: 21.5
Magnum .91XLS: 26.5
OS .61/.91FX: 26.5
Saito .91 19.8
Saito .72 18.2
Every .46 size engine I have is in the 16.7-17.5 oz range. I find a Saito .72 or .91 is the perfect nose weight to add to a tail heavy 3D plane. For totally light weight, simplicity and on a hot twin, you can't beat a 2-stroke. For torque, lugging power and that sweet sound I like on some planes, you can't beat a 4-stroke.